add: add test_failover.py to test failover openai->gemini

Author: jinsoo

tests/test_failover.py

@@ -0,0 +1,26 @@
+# tests/test_failover.py (예시)
+import pytest
+from unittest.mock import patch, MagicMock
+from openai import RateLimitError
+
+def test_failover_on_429():
+    from src.llm_failover_router import LLMFailoverRouter, build_openai_provider, build_gemini_provider
+    from src.config import Config
+
+    # OpenAI provider가 항상 429를 던지게 mock
+    def failing_openai_sync(*, messages, **kwargs):
+        raise RateLimitError("rate_limit", response=MagicMock(status_code=429))
+
+    async def failing_openai_async(*, messages, **kwargs):
+        raise RateLimitError("rate_limit", response=MagicMock(status_code=429))
+
+    from src.llm_failover_router import Provider
+    openai_provider = Provider("openai", failing_openai_sync, failing_openai_async, tier=0)
+    gemini_provider = build_gemini_provider(Config.GEMINI_API_KEY, Config.GEMINI_MODEL)
+
+    router = LLMFailoverRouter(providers=[openai_provider, gemini_provider])
+    result = router.chat(messages=[{"role": "user", "content": "hi"}])
+    assert result  # Gemini 응답이 와야 성공
+    
+if __name__ == "__main__":
+    test_failover_on_429()